1. Field of the Invention
The present invention relates an image heating apparatus which heats a toner image on a recording material. The image heating apparatus includes a fixing apparatus which fixes a toner image onto a recording material and a gloss improving apparatus which heats a toner image fixed onto a recording material to improve the glossiness of the image. The image heating apparatus can be used for an image forming apparatus, such as a copying machine, a printer, and a facsimile, using electrophotography.
2. Description of the Related Art
Various kinds of image forming apparatuses have been known. Among them, the image forming apparatus using electrophotography which subjects a photosensitive member to image exposure with a laser beam and then develops it to obtain an image has been typically widespread. Such image forming apparatus has the advantages in high image quality and high speed and has been widely used as an output unit of a copying machine and a color laser beam printer.
A color image forming apparatus such as the copying machine and color laser beam printer is required to have high productivity (the number of printed sheets per unit time) on various sheets (recording materials) such as a thick sheet.
In order to increase high productivity, in particular, productivity on a sheet having a large basis weight in the image forming apparatus to which electrophotography is applied, the fixing speed of the fixing apparatus need to be increased. A large amount of heat is lost on the sheet having a large basis weight. The amount of heat necessary for fixing on the sheet having a large basis weight is much larger than that for fixing on a thin sheet. At present, the sheet having a large basis weight is subjected to fixing processing by decreasing the fixing speed.
Techniques which can improve the fixing speed of a color image have been already proposed in Japanese Patent Application Laid-Open (JP-A) Nos. 10-149044 and 2005-316421.
The fixing apparatus according to JP-A No. 10-149044 has a pair of fixing members rotatably disposed so as to be pressed into contact with each other, wherein nip portions formed by the pair of fixing members nip and convey a recording material formed with an unfixed image, thereby heating and fixing the unfixed image on the recording material. One or more external heating members can be brought into or out of contact with the pair of fixing members.
As shown in FIG. 10, in the fixing apparatus according to JP-A No. 2005-316421, external heating rollers 53 and 54 are pressed into contact with the surface of a fixing roller 40 to externally heat the fixing roller 40. Both end portions of the two external heating rollers 53 and 54 are rotatably supported by a support frame 30. The support frame 30 is mounted on a pressure arm 32 rotatable on a shaft 34. A compression spring 33 is mounted between the support frame 30 and the pressure arm 32. The pressure arm 32 is mounted so as to be pressed by the compression spring 35. The external heating rollers 53 and 54 are pressed into contact with the fixing roller 40. The external heating rollers 53 and 54 heat the fixing roller 40.
There are provided a mechanism configured to bring a plurality of external heating rollers into and out of contact with the surface of a member to be heated, and an adjusting unit which uniformly brings the plurality of external heating rollers into contact with the surface of the member to be heated.
The related art has the following problems. The fixing apparatuses according to JP-A Nos. 10-149044 and 2005-316421 realize high productivity on a thick sheet as the recoding material which is substantially equal to that on a thin sheet. It is contemplated that the amount of heat of the external heating rollers is increased to increase the amount of heat provided from the external heating rollers to the surface of the member to be heated. To provide a sufficient amount of heat from the external heating rollers to the fixing roller, roller nips which conduct the amount of heat are preferably increased.
The increased diameters of the external heating rollers to increase the roller nips result in making the fixing apparatus larger. Various members such as a cleaning unit and a temperature detecting unit are arranged around the fixing roller. The size of the external heating rollers is limited. When the plurality of external heating rollers are arranged as in JP-A No. 2005-316421, roller nips having a sufficient width are preferably formed by the external heating rollers having a small diameter.
To form large roller nips by the external heating rollers 53 and 54 having a small diameter in the heating and fixing apparatus having the external heating rollers of FIG. 10, the external heating rollers 53 and 54 need to be pressed onto the fixing roller 40 at a high contact pressure. Members having a high spring pressure are used as the springs 33 and 35.
When the external heating rollers 53 and 54 are pressed onto the fixing roller 40 at a high contact pressure, a new problem arises at the attachment of an external heating unit in an assembling process and at the detachment of the external heating unit in maintenance.
The operability is deteriorated because the member under a high spring pressure need to be detached. In the example of FIG. 10, in the state that the spring 35 having a high spring pressure is compressed, a member 70 which supports this is fixed to the fixing apparatus body by a screw. When the screw is unscrewed under the spring pressure of the spring 35, the screw is turned in the state that a pressing force is applied to the thread of the screw. Screw galling is easily caused.
An exclusive jig which compresses the springs 33 and 35 to detach the external heating unit is used. The exclusive jig is necessary for detaching the external heating unit, thereby needing to improve the operability.